For recent video cameras, a high picture quality is demanded together with the operability and the mobility, and in order to satisfy the demand, also those image pickup elements of a small size having a high resolution are prevailing. Together with this, a high performance zoom lens which has a high aperture ratio, a small size and a light weight and has a high magnification is demanded strongly. Further, it is preferable to reduce the number of component lenses and reduce the price of such a zoom lens as described above while a high performance is maintained.
However, a zoom lens of a high magnification generally has a large lens diameter and a large total lens length and besides requires a great number of lenses in order to achieve further strict aberration correction. Therefore, a high magnification zoom lens is generally liable to be large, heavy and expensive, and most high magnification zoom lenses are not suitable for video cameras for consumers.
As a zoom lens used principally for video cameras for consumers, a four-group inner focus type zoom lens is adopted popularly wherein first to fourth lens groups having positive, negative, positive and positive refractive powers, respectively, are disposed in order from the object side and the positions of the second and fourth lens groups are movable. An application of the four-group inner focus type zoom lens to a three-plate type video camera is disclosed, for example, in Japanese Patent Laid-open No. Hei 7-199069 and Japanese Patent Laid-open No. Hei 6-337353.
In particular, Japanese Patent Laid-open No. Hei 7-199069 discloses a zoom lens wherein the third lens group is formed from a single convex lens to form a light beam into an almost afocal or rather divergent light beam which is introduced into the fourth lens group while the fourth length group is formed from three lenses including a cemented lens of a concave lens and a convex lens and another convex lens and is configured so as to be convenient to displace the image side principal point of the fourth lens group to the image side, whereby a long back focus is assured with a small number of component lenses.
Meanwhile, according to the zoom lens disclosed in Japanese Patent Laid-open No. Hei 6-337353, the refractive power of the third lens group is set lower so that a divergent light flux is introduced into the fourth lens group to make the back focus longer, and the fourth lens group is configured such that the image side principal point cannot be displaced toward the image and does not contribute to assurance of the back focus. However, it is considered that the fourth lens group performs correction of chromatic aberration rather excessively to correct magnification chromatic aberration. It is considered that, since the axial chromatic aberration is corrected extremely excessively because correction of the magnification chromatic aberration is performed by the fourth lens group, the correction of the axial chromatic aberration by the fourth lens group is suppressed from becoming excessive by such arrangement of the materials that the third lens group positioned in the proximity of the iris generates axial chromatic aberration.
In a three-plate type video camera, a long back focus for allowing a color separating prism having a great thickness (having a great length in the axial direction) to be disposed behind the lenses is required. Further, since the spectral characteristic of a dichroic mirror for color separation has an angle dependency, it is necessary to set the exit pupil to a state near to a telecentric state so that a uniform spectral characteristic may be obtained over an entire screen. If it is tried to implement a long back focus and a telecentric exit pupil, then the overall length is liable to increase similarly to the zoom lens disclosed in Japanese Patent Laid-open No. Hei 6-337353.
Further, in a three-plate type video camera, since three image pickup elements are used, the price increases when compared with a single-plate camera, and it is demanded that the three-plate type video camera naturally exhibit a high picture quality as is demanded for a model of a high price.
A three-plate type video camera catches images individually of the three primary colors of R, G and B by image pickup elements and superposes resulting electric signals to produce a color image. Therefore, registration adjustment of adjusting relative positions of the image pickup elements is significant, and if the registration adjustment is not sufficient, then a color which the image pickup subject does not have appears at a contour portion of the image or the like. However, even if the registration adjustment is performed with a sufficiently high degree of accuracy, if the image pickup lens has some magnification chromatic aberration, then color offset similar to that which appears when the registration adjustment is insufficient appears and degrades the quality of the image. For the picture quality expected for the three-plate type video camera, it is significant that not only the MTF value is higher than that of the single-plate type video camera but also particularly the magnification chromatic aberration is corrected well.
With the zoom lens disclosed in Japanese Patent Laid-open No. Hei 7-199069, the degree of freedom in chromatic aberration correction is low because the third lens group is formed from a single lens, and it is difficult to correct both of the axial chromatic aberration and the magnification chromatic aberration well.
The present invention has been made in view of such a situation as described above, and it is a subject of the present invention to provide a zoom lens which can be formed with a reduced overall length while physically necessary factors including a long back focus for allowing arrangement of a color separating prism therein and a telecentric property and besides can correct various aberrations including the chromatic aberration to a level suitable for an expensive three-plate type video camera and which includes a comparatively small number of component lenses and an image pickup apparatus which uses the zoom lens.